Effects of Reclaimed Asphalt Pavement on Mechanical Characteristics of Asphaltic Mixtures for Surface Layer

. Reclaimed asphalt pavement (RAP) is considered one of the valuable alternatives to raw materials due to reducing the need to use raw materials, which are less in some world regions. It additionally reduces the highly-priced new bitumen required inside the asphaltic mixture manufacturing and contributes to the preservation of natural resources. To achieve maximum benefit from the integration RAP in asphaltic mixture, it is necessary to investigate the advantages and disadvantages of the recycling process on the properties of asphalt pavement. This study examines the effect of adding reclaimed asphalt pavement by different percentages on the mechanical properties of asphaltic mixture for the surface layer in terms of Marshall's stability, Retained Marshall stability, indirect tensile strength, and compressive strength. Two types of asphalt grade (40-50) and (60-70) were used in addition to one type of aggregate gradation of the wearing course to prepare the asphaltic mixture. The Superpave system was applied to select the best aggregate gradation and optimum asphalt content using Superpave Gyratory Compactor (SGC) and to prepare compacted asphaltic specimens of 100 mm diameter for simulating Marshall’s molds. PAP is added by four different percentages of (7, 13, 19, and 25) % by the weight of the total asphalt mixture, and samples are prepared to compare the mechanical properties with conditional ones. The results show that adding RAP to the asphalt mixture improved the measured properties. In contrast, the mix containing RAP showed lower loss of stability, lower loss in indirect tensile strength, higher stripping resistance, and better durability than the mixture without RAP.


MATERIALS AND TESTING 2.1 Bitumen
Two types of asphalt grade were used in this research: (40-50) and (60-70) which were brought from Al-Doura refinery in Baghdad.Physical tests were conducted and conformed to the limits of Iraqi specifications [18] and ASTM requirements [19].The properties results were successful, as shown in Table 1.AASHTO M 323 specification [20] for calculating the amount of RAP used in the asphalt mixture indicated that the binder must be softened when using a RAP ratio of more than 15%, as shown in Table 2. Soft binder is more expensive; contractors do not want to pay additional amounts.For this reason, asphalt type 60-70 was used because it is suitable for high PAP ratios in super pave mixtures.[20].

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Follow recommendations from blending charts

Aggregate
The crushed aggregate is brought from Al-Sedour quarry.It is commonly used in local road works.Table 3 shows some of its physical properties and conformity with the limits of ASTM specifications.

Mineral Filler
Limestone dust was used as a filler material in this research, and it was brought from the lime factory in Karbala Governorate.The physical properties of the limestone dust are shown in Table 4.

Reclaimed Asphalt Pavement (RAP)
The RAP was brought from one of the projects for the Mayoralty of Baghdad located in the Al-Talbiya area in Baghdad.An extraction test was performed on the RAP according to AASHTO T 164-01 [21].The extraction test showed that the percentage of asphalt present in the RAP mixture was 4 percent, and the gradation of aggregates is shown in Table 5.

Mix Design
The gradation of the aggregates was designed to calculate the optimum asphalt content according to the superpave system.The gradation of the aggregates was selected according to the Iraqi specifications for surface layer SCRB, R9, and the superpave system.Figure 1 shows the selected gradation of the aggregates.
The percentage of optimum asphalt content was adopted according to 4 percent air voids, and it was concluded that the optimum asphalt contents were 4.8% and 4.7 % for (40-50) and (60-70) asphalt grades, respectively.Other properties of the asphaltic mixture were examined at the designed asphalt ratio to ensure that it conforms to the specification AASHTO M323-12.Table 6 shows the properties of the asphaltic mixture for (40-50) and (60-70) asphalt grades.

RESULTS AND DISCUSSIONS 3.1 Marshall Properties
Figures 2 and 3 show the effect of RAP percent on flow and stability at optimum asphalt content (opt.AC) and plus 0.5% of optimum asphalt content (+0.5% Opt.AC), respectively.It is observed from Figure 2 that the value of flow is decreased gradually with increased RAP percent.Also, it can be noted that when the amount of asphalt added to the mixture is inserted, the amount of flow will also decrease with the increase in the amount of RAP added to the mixture.Figure 3 shows that the stability values increase with increasing the RAP contents added to the asphaltic mixture, where the highest stability value is achieved at 25% RAP.It is noticeable that these values are significantly lower when the optimum amount of asphalt is increased by 0.5%.

Retained Marshall Stability (RMS)
Figures 4, 5, and 6 illustrate the RMS with the immersion days.It is noted that these values are reduced when the water periods are increased from 1 to 7 days.It is found that RS values at the opt.AC is higher than values obtained when the amount of asphalt increases by 0.5%.This result is caused by water that attacks the adhesive bond between bitumen and aggregates in the asphaltic mix (stripping), leading to loss of cohesion (strength) and reduced mixture stiffness.It can be seen from Figures 7 and 8 that the RMS values decreased for all RAP percentages along immersion periods.Also, it is noted that RS increased when RAP content increased during one immersion period.Marshall Test results showed that adding PAP to the asphaltic mix improved Marshall Stability and reduced the loss of stability compared with conventional mixes.The reason may back to RAP hardening the asphalt mixture, which leads to an increase in stability because of an increase in the viscosity of the asphalt.It is noted that hardened asphalt had a higher viscosity than the origin asphalt; therefore, it would be less affected by the hot water when subjected to immersion, increasing RAP mix's stability.

Indirect Tensile Strength Test (ITS)
Figures 9 and 10 present the effect of the RAP adding on the results of (ITS) for the unconditioned sample at 25ºC and 60ºC for optimum asphalt ratio and + 0.5 percentage of optimum asphalt.The results have been compounded by the fact that ITS values increase with RAP increases.However, ITS values decrease with asphalt content increasing in the mixture.

Figures 11 and 12
show the results of ITS at 25ºC and 60ºC for optimum asphalt content and +0.5 plus the optimum asphalt ratios for the conditioned sample.The results show a decrease in its values when it is exposed to water, but it increases when the amount of RAP is increased, and it is found that the ITS values at the optimum asphalt content are higher compared to that contain +0.5 opt.AC of the mixture.

Tensile Strength Ratio
The results of the tensile strength ratio for asphaltic mixes at (opt.AC and +0.5 opt.AC) at 25ºC and 60ºC are shown in Figures 13 and 14, respectively.Results show that TSR is higher for mixes containing RAP than the original mix, and these values are increased with RAP increasing until 19% and then begin to decrease when the amount of RAP reaches 25%.Also, there is a decrease in TSR when the percent of asphalt content in the mixture is increased.It can be observed that asphaltic mixtures containing RAP with different percentages showed lower loss of TSR than control mixes.The reason may be that RAP harden the asphalt mixture leads to increasing viscosity over time.However, asphaltic mixes with higher viscosity would play a better role when exposed to tension force and could show a lower reduction in tensile strength under moisture and high temperature.This result was consistent with the results of Fattah et al. [22].

Immersion-Compression Results
Figures 15 and 16 illustrate the compressive strength result for unconditioned and conditioned samples at (opt.AC and +0.5 opt.AC).The values of compressive strength at +0.5 opt.AC is higher than the values at opt.AC for an unconditioned sample because increasing the amount of asphalt will decrease the air voids and increase the density of the mixture, while the value at +0.5 opt.AC is decreased from the value of opt.AC for a conditioned sample because asphalt is affected by temperature causing softness in the mixture.

Index of Retained Strength (IRS)
Results of IRS at (opt.AC and +0.5 opt.AC) are shown in Figure 17.It was found that IRS values increased as RAP content increased and decreased when the amount of asphalt content increased in the mixture.

CONCLUSIONS
Based on the obtained test results, the following conclusions can be drawn: • The Marshall's stability value is higher for the asphaltic mixture contained RAP percentage than the control mixture.These values gradually increase as the added RAP ratio increases to the mixture, reaching the highest value at 25% percent by weight of the total mixture.• The Retained Marshall Stability (RMS) values for all RAP percentage decreased along immersion periods.The RMS values increased with RAP content increasing for 1 day immersion, and the RMS values for opt.AC is higher than mixture with +0.5 opt.AC. • The indirect tensile strength for asphaltic mixtures at optimum asphalt content and +0.5 optimum asphalt content increased with RAP percentage for conditioned and unconditioned samples, and the higher value was obtained when the weight of total mix adds 19% of RAP content.• The values of compressive strength increased with increase RAP percent.The value at +0.5 opt.AC is higher than the value at opt.AC for unconditioned sample, while the value at +0.5 opt.AC is decreased from the value of opt.AC for conditioned sample.• The index of retained strength value gradually increased when the RAP increased and decreased when the amount of asphalt content added to the mixture increased by 0.5 percent.

Figure 1 :
Figure 1: Gradation of the selected blend with specifications.

Figure 2 :
Figure 2: Relation between flow and RAP content at opt and +0.5 opt.AC.

Figure 3 :
Figure 3: Relation between stability and RAP content at opt and +0.5 opt.AC

Figure 7 :
Figure 7: Comparison between RMS values for RAP percentages at Opt. AC.

Figure 9 :
Figure 9: ITS for unconditioned sample at 25 ˚C.Figure 10: ITS for unconditioned sample at 60˚C.

Figure 10 :
Figure 9: ITS for unconditioned sample at 25 ˚C.Figure 10: ITS for unconditioned sample at 60˚C.

Figure 11 :
Figure 11: ITS for conditioned sample at 25˚C.

Figure 12 :
Figure 12: ITS for conditioned sample at 60˚C.

Figure 15 :
Figure 15: Compressive strength and RAP content for unconditioned sample at opt.AC and +0.5 opt AC.

Figure 16 :
Figure 16: Compressive strength and RAP content for conditioned sample at opt.AC and +0.5 opt AC.

Table 2 :
Guidelines for binder selection for RAP mixtures

Table 3 :
Physical properties of used aggregates.

Table 6 :
Mixture properties at optimum binder contents according to superpave system.